The following paragraphs are not an admission that anything discussed in them is prior art or part of the knowledge of persons skilled in the art.
Electrochemical cells are energy conversion devices and usually are used to collectively indicate fuel cells and electrolyzer cells.
Fuel cells have been proposed as a clean, efficient and environmentally friendly source of power that can be utilized for various applications. A conventional proton exchange membrane (PEM) fuel cell is typically comprised of an anode, a cathode, and a selective electrolytic membrane disposed between the two electrodes. A fuel cell generates electricity by bringing a fuel gas (typically hydrogen) and an oxidant gas (typically oxygen) respectively to the anode and the cathode. In reaction, a fuel such as hydrogen is oxidized at the anode to form cations (protons) and electrons by the reaction H2=2H++2e−. The proton exchange membrane facilitates the migration of protons from the anode to the cathode while preventing the electrons from passing through the membrane. As a result, the electrons are forced to flow through an external circuit thus providing an electrical current. At the cathode, oxygen reacts with electrons returned from the electrical circuit and with the protons that have crossed the membrane to form liquid water as the reaction by-product following ½O2+2H++2e−=H2O.
On the other hand, an electrolyzer uses electricity to electrolyze water to generate oxygen from its anode and hydrogen from its cathode. Similar to a fuel cell, a typical solid polymer water electrolyzer (SPWE) or proton exchange membrane (PEM) electrolyzer is also comprised of an anode, a cathode and a proton exchange membrane disposed between the two electrodes. Water is introduced to, for example, the anode of the electrolyzer, which is connected to the positive pole of a suitable direct current voltage. Oxygen is produced at the anode by the reaction H2O=½O2+2H++2e−. The protons then migrate from the anode to the cathode through the membrane. On the cathode, which is connected to the negative pole of the direct current voltage, the protons conducted through the membrane are reduced to hydrogen following 2H++2e−=H2.
A typical electrochemical cell employing PEM comprises an anode flow field plate, a cathode flow field plate, and a membrane electrode assembly (MEA) disposed between the anode and cathode flow field plates. Each reactant flow field plate has an inlet region, an outlet region, and open-faced channels to fluidly connect the inlet to the outlet, and provide a way for distributing the reactant gases to the outer surfaces of the MEA. The MEA comprises a PEM disposed between an anode catalyst layer and a cathode catalyst layer. A first gas diffusion layer (GDL) is disposed between the anode catalyst layer and the anode flow field plate, and a second GDL is disposed between the cathode catalyst layer and the cathode flow field plate. The GDLs facilitate the diffusion of the reactant gas, either the fuel or oxidant, to the catalyst surfaces of the MEA. Furthermore, the GDLs enhance the electrical conductivity between each of the anode and cathode flow field plates and the electrodes.
In practice, fuel cells are not operated as single units. Rather fuel cells are connected in series, stacked one on top of the other, or placed side-by-side, to form what is usually referred to as a fuel cell stack. The fuel, oxidant and coolant are supplied through respective delivery subsystems to the fuel cell stack. Also within the stack are current collectors, cell-to-cell seals and insulation, with required piping and instrumentation provided externally to the fuel cell stack. Fuel cross-over from the anode side of the fuel cell to the cathode side, or from the cathode side to the anode side, may occur when the seals between cell components are inadequate or when the membrane of the MEA is ruptured. This typically develops as the stack ages. There is generally a small cross-over of hydrogen or oxygen occurring naturally across the MEA (driven by concentration gradients). The net diffusion due to the concentration gradient will be from either anode to cathode or cathode to anode depending on which electrode is at the higher pressure. The presence of fuel on the oxidant side of the fuel cell, or oxygen on the fuel side of the fuel cell, is highly undesirable because of the direct combustion reaction that may take place. The heat generated during this reaction may cause damage to the membrane and/or other parts of the fuel cell. Un-combusted fuel will follow the cathode exhaust stream and mix with the oxygen therein. This is undesirable because of the risk for such a mixture to ignite (4 percent of hydrogen per volume in air is hydrogen's lower flammability limit). In addition, the hydrogen/fuel side of the stack may develop external leaks over time. These need to be avoided or designed into the system so that flammable mixtures are not created in the fuel cell environment. Furthermore, the fuel cell can leak internally due to faulty seals or separator plates, e.g., leakage from reactant fluid to coolant or vice versa.
Fuel cell stacks have been used as power sources in various applications, such as fuel cell powered electric vehicles, residential power generator, auxiliary power unit, uninterrupted power source, etc. For fuel cell stacks to be used in power generation applications, many peripheral devices, conditioning devices are needed since fuel cell stacks rely on peripheral preconditioning devices for optimum or even proper operation. Extensive piping and plumbing work is also required for connection between such devices.
For example, in the situation where the fuel gas of the fuel cell stack is not pure hydrogen, but rather hydrogen containing material, e.g., natural gas, a reformer is usually required in the fuel delivery subsystem for reforming the hydrogen containing material to provide pure hydrogen to the fuel cell stack. Moreover, in the situation where the electrolyte of the fuel cell is a proton exchange membrane, since most of the membranes currently available requires a wet surface to facilitate the conduction of protons from the anode to the cathode, and otherwise to maintain the membranes electrically conductive, a humidifier is usually required to humidify the fuel or oxidant gas before it comes into the fuel cell stack. In addition, most conventional fuel cell systems utilize several heat exchangers in gas and coolant delivery subsystems to dissipate the heat generated in the fuel cell reaction, provide coolant to the fuel cell stack, and heat or cool the process gases. In some applications, the process gases or coolant may need to be pressurized before entering the fuel cell stack, and, therefore, compressors and pumps may be added to the delivery subsystems.
These peripheral devices as well as the fuel cell stacks are often packaged together as a power module (fuel cell power module, FCPM), which will often be disposed in a confined environment, where space is limited, such as vehicular applications or other portable applications. Control and regulation of the FCPM is typically performed by an electronic control unit (ECU), which forms an integral part of the FCPM.
Therefore, there is a need for an electrochemical cell system that provides an indication of leaks in the cell stack or out from the cell stack to an operator of the system.